In the prevalence study of healthcare-associated infections, the following lengths of stay per infection were observed: 10, 4, 4, 1, 8, 7, 18, 25, 18, 9, 5, 18. What is the mode?

• A. 10
• B. 4
• C. 18
• D. 25

Answer: (C)

The mode is the value that appears most often in a data set. In this list, 18 shows up three times, more frequently than any other number (the next most common is 4, which appears twice; all other values appear once). So the most common length of stay per infection in this sample is 18, making 18 the mode. The mode highlights the most typical observation, though it doesn’t describe overall spread or the average.